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Reviewer: Pavol Navrat

So far, five books have been published in the "Pattern-Oriented Software Architecture (POSA)" series?this is the fourth volume. Frank Buschmann is listed as one of the coauthors in all but the third volume. The subtitle of this volume reflects the scope of patterns that Buschmann, Henney, and Schmidt attempt to deal with. Their focus is on the design and implementation of software for distributed computing systems. The intended focus undoubtedly covers a broad range of typical distributed computing problems. This book explains, in fact, 114 patterns concerned with distributed computing, and provides links to more than 150 patterns presented in other publications. The book has over 600 pages, structured into 22 chapters, and grouped into three parts: "Some Concepts," "A Story," and "The Language." The concepts included are on patterns and pattern languages, distributed systems, and the pattern language used by the authors. The question is raised whether the presented content defines a language in some stricter, not to say formal, sense. A formal language can be described by a grammar or by a Backus-Naur form, or there may be other formalisms that are more suitable for the type of language needed to express patterns. Patterns may require a more complicated kind of language?one not available to a full formalization. It is very likely that to express patterns, we may need a combination of textual and diagrammatic language. But in all these cases, one encounters a certain level of rigor. This does not mean to imply that the book is written without any rigor at all. However, the book's title does suggest that it is going to describe a language. For example, I would expect rules on how to form expressions combining patterns, or expressions describing sentences formed from patterns. To be fair, these questions are not easy to answer and are the subject of current research. My comments are merely motivated by the title, which claims to present a language. Another comment refers to the number of patterns presented (bearing in mind that this is just one of five sequential books). If we have to deal with hundreds of patterns, we are facing questions about the proper level of specialization, lack of hierarchy, and useful categorization. Perhaps the field of pattern-oriented software architecture must undergo some kind of extensive development before a consolidation phase comes, yielding a new synthesis. Despite these comments, which are more on the current state of the field than on the content of the book, this is a respectable work. It is a true contribution to the development of software architectures. Online Computing Reviews Service

Reviewer: Fernando Berzal

This series started more than a decade ago with a landmark work [1] that complemented and completed the quintessential monograph on software design patterns, the Gang of Four book [2]. The second pattern-oriented software architecture (POSA) volume, which focused on the construction of concurrent and distributed systems [3], appeared at the turn of the century. The third installment in this series [4] dealt with resource management topics. This fourth volume, however, has replaced the traditional POSA detailed pattern catalog style with a higher-level overview of design patterns as they apply to the development of complex software systems. The emphasis has shifted from implementation details to the description of the relationships and tradeoffs among alternative and complementary design patterns. The first part of the book sets the context for the volume. As you might expect, the introductory chapters include a brief overview of design patterns, and a concise account of the evolution of distributed systems, from ad hoc sockets to structured communication to complex middleware systems. But, apart from these fundamentals, the authors also stress the importance of studying how design patterns complement and complete each other. From the outset, they let us know that they strive to "strike the right balance between the specific and the general to ensure that the patterns within a language are also documented in a sufficiently complete form individually, but with obvious emphasis on their interconnections." "A Story," the second part of this work, describes the application of pattern sequences to three different areas of a product-line architecture for warehouse management control systems. These pattern stories let readers understand how the application of design patterns affects the design and implementation of complex software systems. I cannot agree more with the authors when they assert that patterns are a "powerful tool for developing software architectures on the basis of thoughtful and explicit design decisions." Their three pattern sequences are instructive and engaging. I would certainly recommend making use of such case studies when teaching software design courses. The bulk of this book, however, is devoted to a pattern language that addresses key areas in the construction of distributed software systems. Some of these areas deal directly with distributed computing issues, though most of them just play important supporting roles in the development of distributed systems. In fact, the latter are often key in the development of any complex software system. This work, therefore, is less focused than its predecessors. It collects patterns from a variety of different sources, and presents them as a single, coherent pattern network. The book groups a whopping set of 114 design patterns into 13 problem areas that developers must usually consider when building complex distributed systems: structural design, distribution infrastructure, events, interface partitioning, component design, application control, concurrency, synchronization, object interaction, adaptation and extension, implementation of state machines, resource management, and database access. Each chapter is focused on one of these key areas, and starts with an interesting discussion describing the tradeoffs addressed by the corresponding design patterns. This introductory material is then followed by short two- or three-page descriptions of every design pattern covered in the book. As mentioned above, these Web-like narrative descriptions are more focused on analyzing the relationships among patterns than on discussing implementation details. The book organizes much of the work that has been published in design pattern form during the last decade. Apart from the material in previous POSA volumes [1,3,4], it integrates the Gang of Four patterns [2], Martin Fowler's work [5], and many other pattern catalogs into a coherent whole. Of course, it does not forget those patterns that are more directly related to the development of distributed systems [6,7]. Due to its broad scope, however, this work omits many details that, from the authors' point of view, might distract readers from the big picture. Unfortunately, this makes this volume less self-contained than other pattern catalogs. The book's style might not be too appealing for a designer apprentice interested in getting acquainted with design patterns and their use in the development of complex software systems, but the work is still a valuable resource, appropriate for the library of any self-respecting software designer. Online Computing Reviews Service

Reviewer: Gonzalo Peralta Villavicencio

This book is intended for students and advanced professionals working on designing distributed systems, specifically those who are trying to express their system's software architectures based on recurrent design solutions. The authors present a pattern language for distributed computing based on their broad experience in the design of distributed systems. The book consists of 22 chapters, divided into three well-defined parts. Part 1 begins by presenting fundamental concepts about patterns and distributed systems, and makes an introductory presentation of the language of patterns. Buschmann et al. cataloged 114 design patterns into 13 problem areas according to their scope, order of relevance, and applicability. These problem areas try to abstract all of the complexity in distributed systems, and constitute the basis upon which the language is constructed. Part 2, "A Story," presents a use case for the pattern language from a real-life system related to industrial automation (a warehouse management process control system). The raised solution to this use case implies the application of patterns in three contexts: baseline architecture, communication middleware, and warehouse topology. The patterns selected reflect design decisions for solving high-level issues related to stock management, order management, shipping, receiving, material flow control, and topology management. Having some experience applying object-oriented (OO) frameworks for networked and concurrent systems will be very useful for following the development of communication middleware in Part 2. Those familiar with patterns and OO toolkits will easily pick up on the challenges related to this very important context. Part 3 treats the language of patterns in a structured way. This formality allows designers to understand the key problem areas, and to identify patterns useful for the design of a distributed system. All presented diagrams are based on unified modeling language (UML), with some variations. The assimilation and organization of the 114 patterns that compose the proposed language make this work one of the most important patterns compendiums. The formal presentation of each pattern in general is sufficient, but those looking for more detailed information about a specific pattern must follow the references cited throughout the book. The authors' decision to include a real-life use case was a good strategy to illustrate the application of the pattern language. As they mention, this is not a definitive guide to design patterns, but I consider it a must for a pattern books collection. Online Computing Reviews Service